The Asexual Cycle of Planaria Velata in Relation to Senescence and Rejuvenescence

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THE ASEXUAL CYCLE OF PLANARIA VELATA IN
RELATION TO SENESCENCE AND REJU-
VENESCENCE.

C. M. CHILD.

I. The Life Cycle under Natural Conditions.

During early spring in the region about Chicago, a planarian
appears in temporary ditches and pools, particularly in those
which are more or less filled with dead leaves. It is also often
found in permanent bodies of water such as springs, permanent
ponds and brooks, but seems to attain the greatest numbers
in the temporary ditches and pools. The animal is apparently
the species recently described by Stringer ('09) and named
Planaria velata. The shape and proportions of the larger
individuals are indicated in Fig. 1.

When the animals first appear soon after the ice melts they
are mostly only 2-3 mm. in length and commonly light in color.
They grow rapidly and soon the dorsal surface becomes very
deeply pigmented so that they appear almost black. They are
very active and their locomotion is much more rapid than that
of most other fresh water planarians. During this period they
react readily to meat of various kinds and can be collected in
large numbers by placing pieces of meat in the water. In about
four weeks they attain a length of 12-15 mm., their movements
gradually become slower, they cease to react to food, become
light gray in color from loss of pigment and sooner or later the
pharynx disintegrates.

Within a few days after these changes a process of division
begins. As the worms creep about, the extreme posterior end
adheres to the substratum and the rest of the animal pulls away
and leaves it behind as a small fragment which becomes more or
less spherical and within a few moments is covered with a slime
which adheres to the underlying surface and hardens into a
cyst. This process of division is repeated, often several times
within a few moments, so that as the animal moves across the

181

1 82 C. M. CHILD.

containing vessel it may leave behind it a series of such pieces.
The pieces vary considerably in size, some being as large as 1.5
mm. in diameter, some only about 0.5 mm. The process con-
tinues until half or two thirds or sometimes even more
of the worm is separated into pieces and then the
anterior region including the head may encyst without
further division or in some cases dies.

Under natural conditions the encysted pieces remain
quiescent during the summer and the following winter
and in early spring emerge from the cysts as minute,
very active worms which at once begin to feed and
grow and repeat the cycle. As I have determined by
experiment, the encysted pieces are not capable of with-
standing desiccation and it is probable that this fact is
connected with the occurrence of the worms in ditches
and pools partly filled with dead leaves. In such locali-
ties even though the water disappears, the bottom under
the thick layer of leaves is always more or less wet and
the encysted pieces are not subjected to drying.

During the last thirteen years I have collected these
worms almost every year and have never found a single
individual with mature sexual organs or even any indi-
cation of sexual reproduction. Every year the active
period ends with decrease in activity, cessation of feed-
1 ing, loss of pigment, fragmentation and encystment of

the fragments.
In this species then, under the conditions where it occurs in
this locality, development and growth result in a process of
senescence, the individual breaks up into fragments which under-
go regulation within the cysts to small whole animals, and these
are to all appearances physiologically as well as morphologically
young and are capable of repeating the life cycle. In short,
senescence is followed in these animals not by death but by asex-
ual reproduction and rejuvenescence.

During a number of years I have kept a stock of these worms
in the laboratory, have bred them through several asexual
generations and have subjected them to various experimental
conditions. The results of this asexual breeding and the experi-

ASEXUAL CYCLE OF PLANARIA VELATA. 1 83

mental modifications of the life cycle will be discussed in another
paper.

II. The Physiological Resistance to Depressing Agents
of Young and Old Worms.

In these experiments the method of comparing resistances
which I have called the direct method was used. Here the
depressing agent is used in sufficiently high concentration to kill
the animals within a few hours and the occurrence of death is
determined by disintegration of the worms which begins within
a few moments after death. This method has been fully de-
scribed in another paper (Child, '13a). In that paper it was
shown that with this method the animals with the higher rate
of metabolism or more strictly of cell respiration are less resistant
and therefore die and disintegrate earlier than those with the
lower rate. Thus the differences in resistance enable us to
compare the rates of respiration and so in a general way the rates
of metabolism.

In Table I. the first vertical column gives the length of time
in the depressing agent in hours and minutes, the second the
serial numbers of the lots of worms compared, and the columns
I.-V. under "Stages of Disintegration" give the number of
worms of each lot in each stage of disintegration at each time.
As regards the five stages, I have found it convenient to distin-
guish more or less arbitrarily these stages in the process of dis-
integration, for disintegration usually appears first in certain
definite regions of the body while other regions are still alive and
show movement and it follows a more or less regular course
(Child, '13a). The five stages are briefly characterized as fol-
lows:

I. Intact, no disintegration.

II. Disintegration beginning, usually in head region.

III. Body beginning to disintegrate but form still retained.

IV. Margins disintegrated, form disappearing in consequence
of swelling of tissues and separation of cells.

V. All epithelium and pigment gone; swelling of tissues has
extended to all parts and original form has disappeared.

The distinction of these stages makes it possible to compare

1 84

C. M. CHILD.

different lots of worms more closely than if only the time of
complete disintegration were noted.

In Table I. Lot I consists of ten worms 1.5-2 mm. in length,
which had emerged from cysts within three or four days preceding
and had been fed once with pieces of earthworm after emergence.
Lot 2 consists of ten worms 13-15 mm. which had been raised
in the laboratory from cysts with earthworm as food and were
almost ready to encyst again.

Table I.

Series 77. KCN 0.001 mol.

Length of Time in

Lots.

Stages of Disintegration.

KCN.

I.

II.

III.

IV.

V.

I

8

I

I

1.30

2

10

I

5

I

I

3

2.00

2

10

I

4

I

5

2.30

2

10

I

2

2

I

5

3-oo

2
I

10

3

7

3-30

2

6

3

I

I

10

4.00

2

3

5

I

I

4-30

2

8

I

1

5.00

2

4

4

I

1

5-30

2

I

4

3

2

6.30

2

3

7

7-30

10

It is evident at once from the table that the resistance of the
worms of Lot 1 recently emerged from cysts is very much less
than that of the large worms of Lot 2. Disintegration begins in

ASEXUAL CYCLE OF PLANARIA VELATA. 1 85

Lot i after one and one half hours in KCN, while the worms of
Lot 2 are still intact and slowly moving about. In Lot 2 dis-
integration begins after three hours. All the worms of Lot I
are completely disintegrated after three and one half hours,
those of Lot 2 after seven and one half hours, i. e., the survival
time of Lot 2 is nearly double that of Lot 1. In other words
the worms of Lot 1 have a much higher rate of metabolism than
those of Lot 2.

That the difference in size of the worms is not responsible
for the difference in survival time is evident for two reasons : first
in these flattened elongated animals the surface increases almost
as rapidly as the volume and second the time of beginning of
disintegration (Stage II.) is much later in Lot 2 than in Lot 1.
The earliest stages of disintegration involve the external surface
of the body and the surface of the large worms including the
cilia remains alive for a much longer time than that of the small
worms. Moreover, if the difference in size determined the dif-
ference in survival time we should expect that this would be
much greater since the small worms are only a minute fraction
of the size of the larger. The difference in the rate of the meta-
bolic processes affected by the KCN is the only factor which
will account for the results (Child, '13a).

Unfortunately it has thus far been impossible to compare the
worms emerging from the cysts with young worms hatched from
eggs because I have never observed sexual reproduction in this
species, but the difference in rate of metabolism between the
small and large worms is similar to the difference known to exist
in other forms between young animals sexually produced and old.

It is, however, not necessary to use the extremes of the life
cycle for comparison. Animals in various stages of growth may
be compared and in all cases those which are nearer the stage
when encystment occurs, i. e., those which are older as regards
growth and development, show the higher resistance.

In Table II. the survival times of a series consisting of five
worms 5-6 mm. in length (Lot 1) and five worms 11-12 mm.
(Lot 2) are given.

186

C. M. CHILD.

Table II.

Series 64, I., II.

KCN 0.001 mol.

Lots.

Stages of Disintegration.

KCN.

I.

II.

III.

IV.

V.

2.30

I

2

2

5

3

3.00

I
2

5

3

I

I

3-30

I
2

3

I

2

2

I

I

4.00

I
2

1

4

I

4

4-30

I
2

4

I

S

5-30

2

s

6.30

2

4

1

7-30

2

5

In this series also the younger worms show less resistance,
which signifies a higher rate of metabolism, but a comparison of
Table II. with Table I. shows that Lot I of Table II. has a longer
survival time than Lot I of Table I., i. e., worms of 5-6 mm. in
length have a lower rate of metabolism than worms recently
emerged from cysts. These facts show that a progressive de-
crease in the rate of metabolism occurs during the growth of the
animals. Those newly emerged from cysts have the highest
rate, those which are full-grown and nearly ready to fragment
and encyst have the lowest rate, while intermediate stages show
rates between these two extremes.

These results have been confirmed by various other series with
both KCN and alcohol. The small newly emerged worms die
much earlier in all cases than the large worms. The differences
in resistance to KCN, alcohol, etc., between young and old
animals are the same in animals freshly collected from their
natural habitat as in animals bred for one or more generations
in the laboratory.

ASEXUAL CYCLE OF PLANARIA VELATA.

I8 7

The results obtained in this way are further confirmed by the
much greater activity of the small recently emerged animals.
They move much more rapidly, are much more irritable and show
a much higher rate of growth than the large animals. And
finally the small worms from the cysts are capable, as noted in
the preceding section, of repeating the life cycle. There can I
think be no doubt that the worms emerging from the cysts are
physiologically young and that they undergo a process of senes-
cence as they grow in size. Evidently a process of rejuvenescence
is associated in some way with the asexual reproduction which
follows growth and development.

III. Experimental Reproduction.
1. The Course of Experimental Reproduction.

The process of reproduction of whole animals from pieces
isolated by section is very similar to that in other planarians.
Pieces from any region of the body and above a certain limit of
size, which varies somewhat with the region, are capable of giving
rise to whole animals.

As in other species of Planaria, the process consists in part of
the outgrowth and differentiation of embryonic tissue from the
cut surface and in part of redifferentiation of other tissues to a
greater or less distance from the cut surface. In pieces of equal
length the amount of anterior new tissue is greater and of
posterior new tissue less in those from the anterior region of the
body, while with increasing distance of the end of the piece from
the head region the amount of anterior new tissue increases and

3

1 88 C. M. CHILD.

that of posterior new tissue decreases. The development of the
new head is more rapid in anterior than in posterior pieces. The
position of the new pharynx is posterior to the middle in anterior
and anterior to the middle in posterior pieces. Short pieces
from the extreme anterior region frequently fail to develop a
new posterior end. Fig. 2 shows a piece of this kind. Figs. 3,
4 and 5 show three pieces, the first from the anterior, the second
from the middle and the third from the posterior region. The
different amounts of new tissue produced are seen in the figures.
All these graded differences, like those in Planaria dorotocephala
(Child, 'lie), indicate the existence of a physiological gradient
of some sort along the axis. As a matter of fact this gradient is
essentially similar to that which exists in P. dorotocephala (Child,
'12, '13c).

2. The Encystment of Artificially Isolated Pieces in Relation to
Size of Piece and Region of Body.

Pieces isolated by section may undergo the regulation to whole
animals either with or without encystment. The frequency of
encystment varies with region of the body from which the piece
is taken, with the size of the piece and with the physiological
age of the animal. The following records of series will serve to
illustrate this. In these series a number of worms, ten, twenty
or twenty-five, from the same stock and as nearly as possible of
the same size and in the same physiological condition are cut
into a number of as nearly as possible equal pieces, the corre-
sponding pieces are placed together in one lot and results recorded
for each piece. Since different numbers of worms are used in
different series the results are given in percentages.

Series ig, April 13, ign. — Ten worms, full grown (12-14
mm.), but still feeding and deeply pigmented. Heads removed
and remainder of body cut into two equal pieces, a, the anterior,
and b, the posterior. Table III. shows the percentages of the
pieces which develop into whole worms without encystment
and of pieces which encyst soon after the operation and emerge,
from a few days to several weeks later, as whole worms after
regulation in the cysts.

ASEXUAL CYCLE OF PLANARIA VELATA. 1 89

Table III.

No Encystment. Encystment.

a 90 10

b 60 40

Series 20, April 13, iqii. — Ten worms from same stock, of
same size and in same condition as Series 19. Heads removed
and body cut into four equal pieces, a, b, c, d, a being the most
anterior. Table IV. gives the results.

Table IV.

No Encystment. Encystment.

a 90 10

b 80 20

c 30 70

d 20 80

Series 27, April 17, iqii. — Ten worms like those of Series 19
and 20 in size and condition. Heads removed and body cut into
eight equal pieces, a-h, a being the most anterior. The results
are given in Table V.

Table V.

No Encystment. Encystment.

a 20 70

b 10 90

c 100

d 100

« 100

/ 100

g 100

h 100

It is evident from these three series and abundantly confirmed
by numerous others, first, that the frequency of encystment of
pieces increases from the anterior to the posterior end of the body
and second, that the frequency of encystment increases as the
size of the piece decreases. In all these series the greater fre-
quency of encystment in more posterior pieces is evident in
greater or less degree. In Series 19 where the pieces represent
halves of the body the percentages of encystments are small,
in Series 20, composed of \ pieces, they are larger except in the

190 C. M. CHILD.

most anterior piece and much larger in the two posterior pieces
c and d which together equal b of Series 19. And finally, in
Series 27 which consists of ^ pieces all the pieces encyst except
30 per cent, of a and 10 per cent, of b. When the pieces are
cut still smaller all encyst.

The frequency of encystment then shows in pieces of equal
size a gradation from the anterior to the posterior end of the
body and indicates the existence of some sort of a physiological
gradient in the animal. Encystment may, however, occur in
pieces from any region if they are sufficiently small, but in general
anterior pieces must be smaller than posterior pieces to give the
same frequency of encystment. This fact indicates that the
physiological state of the piece differs in some way with its size.
As a matter of fact this species possesses essentially the same sort
of gradient in rate of metabolism as Planaria dorotocephala
(Child, '13a, '13c) and the relation between frequency of encyst-
ment, region of the body and size of the piece depends upon the
existence of this gradient and the changes in rate of metabolism
which occur in pieces from different regions of the body and of
different size after isolation. Further consideration of these
points is postponed to another time.

3. The Frequency of Encystment of Pieces in Relation to
Temperature.

Series 53, October 5, 1911.— Animals 9-10 mm. in length were
selected from a stock which had been kept at a temperature of
20 C, the heads removed and the bodies cut into four equal
pieces a-d. Lots of ten each of each of the four pieces were
placed in three different temperatures, io°, 20 and 28-30 C.
Table VI. gives the results in percentages.

It is evident at once from Table VI. that the frequency of
encystment is greater with higher than with lower temperature,
i. e., the higher the rate of metabolism in the pieces the greater
the frequency of encystment. Numerous other series give the
same results without exception, not only for pieces, but for whole
worms. Worms which have been kept at a temperature of 20 ,
when placed in a temperature of 30 will often encyst entire
while at 20 they remain active until they fragment and the
pieces encyst, and at io° many of them do not encyst at all.

ASEXUAL CYCLE OF PLANARIA VELATA.

191

Table VI.

Temperature.

Pieces.

No Encystment.

Encystment.

Dead.

a

100

10°

b
c

90

20

10

80

d

10

90

a

100

2 0°

b
c
d

a

70

30
100
100

100

28-30°

b
c

100
100

d

100

4. The Frequency of Encystment in Pieces in Relation to Age.

In the very small young worms recently emerged from cysts
pieces, unless very small, usually reproduce whole worms with-
out going through a period of encystment. As the worms in-
crease in size and become physiologically older the frequency of
encystment increases until in worms which are almost ready to
fragment and encyst naturally all pieces resulting from section
usually encyst. .

The differences in this respect between half grown worms,
worms which are about full grown but have not yet ceased to
feed and still retain their dark color and worms which have
stopped feeding and become gray in color are shown in the three
series following.

Series 47, September 21, iqii. — Twenty worms about half
grown (7 mm. in length) were cut into four equal pieces, a-d.
The percentages of regulation without encystment and of en-
cystments appear in Table VII.

Table VII.

No Encystment.

Encystment.

Dead.

95
95
55

20

5
45
80

5

b

d

192 C. M. CHILD.

Series 56 I, October 12, iqii. — Ten worms full grown but still
dark in color and still feeding. Body cut into four equal pieces,
a-d. Table VIII. gives percentages of encystments.

Table VIII.

No Encystment. Encystment.

a 60 40

b IOO

c IOO

d IOO

Series 58 I, October 13, iqii. — Ten worms, full grown, gray in
color and no longer feeding. Body cut in four equal pieces,
a-d. Table IX. gives percentages.

Table IX.

No Encystment. Encystment.

« IOO

b 100

C IOO

4 100

The older worms show the greater frequency of encystment of
pieces. The same results have been obtained in other similar
series without exception.

5. The Physiological Condition of Animals Reproduced from
Artificially Isolated Pieces.

The animals reproduced from pieces isolated by section are
physiologically young, whether a period of encystment occurs or
not. In this respect they are similar to the worms produced
from the pieces which separate and encyst naturally. Small
pieces cut from the bodies of old worms and allowed to repro-
duce whole animals show the same differences in rate of metab-
olism from old animals as the worms emerging from cysts
naturally produced. The differences in susceptibility to cyanide
are essentially the same as in Table I. Moreover, these small
worms arising from pieces of large old worms are capable of
rapid growth if fed and of repeating the life cycle. As they grow
the rate of metabolism, as indicated by their susceptibility to

ASEXUAL CYCLE OF PLANARIA VELATA. I93

cyanide, decreases, the rate of growth and the degree of activity
also decrease, they finally stop feeding, lose their dark color and
give rise to cysts again and from these a new generation of young
worms emerges. Stocks of animals produced from pieces have
passed through this cycle repeatedly in the laboratory.

The degree of rejuvenescence in this experimental reproduc-
tion varies with the size of the piece. The smaller the piece, the
more extensive the reorganization and the younger the worm
which results. In all respects these results are essentially the
same as those obtained with Planaria dorotocephala and described
in an earlier paper (Child, 'wb).

It is evident also that there is no essential difference in this
respect between the process of fragmentation in old worms and
the reproduction of young worms from the encysted pieces in
nature and the process of experimental reproduction of animals
from pieces isolated by section. In nature the fragmentation
occurs only in old animals by a process characteristic of a certain
stage of the life cycle. In experiment the pieces can be isolated
at any stage of the life history and may be of any size. In both
cases the reorganization, together with the period of starvation
which is also a factor as will appear, brings about rejuvenescence
and the worms thus produced are capable of repeating the life
history from the stage at which they begin again to feed to the
stage of fragmentation.

IV. The Nature of the Process of Encystment.

It has been shown that the frequency of encystment of pieces
increases with rising temperature, with decreasing size of the
piece, with increasing distance of the level of the piece from the
head region and with advancing age of the animals. Pieces from
any region of the body may encyst if the temperature is suffi-
ciently high, if the pieces are sufficiently small or if the animal is
sufficiently old. All of these conditions must have something in
common as regards their effect upon the pieces since all produce
similar results. What is this common factor?

When a piece is cut from the body it is stimulated and its rate
of metabolism increases. This is generally admitted but it can
also be demonstrated by the cyanide method. The suscepti-

194 c - M - CHILD.

bility to cyanide of a piece immediately after isolation is much
greater than that of the corresponding region of the body in an
uninjured animal of the same age and physiological condition.
This greater susceptibility of the piece means that it has been
stimulated by the act of isolation. After this sudden rise its
susceptibility to cyanide decreases gradually during twenty-four
hours or more and in small pieces may fall below that of corre-
sponding regions in the uninjured animal (Child, '13&). This
decreasing susceptibility means that the rate of metabolism in
the piece is gradually decreasing as the stimulation resulting from
section gradually disappears.

The cyanide method shows further that the degree of stimula-
tion increases as the size of the piece isolated decreases and also
as the distance of the level of the piece from the head region
increases. In other words smaller or more posterior pieces are
more stimulated by the act of section than larger or more anterior
pieces. And finally pieces cut from worms at a higher tempera-
ture within certain limits are more stimulated and show a
greater increase in rate than pieces from worms at a lower
temperature.

These relations between the degree of stimulation of pieces
and the factors of size of piece and region of the body and various
external conditions have been worked out completely for Planaria
dorotocephala and the data will be presented in full elsewhere.
Sufficient work has been done on P. velata to show that the
relations are essentially the same as in P. dorotocephala, but since
the work on the latter species furnishes the foundations for the
conclusions and since the data for that species are in more com-
plete form and will be published in a short time the evidence for
the above statements concerning the degree of stimulation in
pieces of P. velata is not presented in detail.

So far then as region of the body, size of piece and temperature
are concerned the frequency of encystment of pieces in P. velata
runs parallel to the degree of stimulation by the act of section.
Apparently the more the piece is stimulated by section the more
likely it is to encyst.

The process of encystment in this species consists in the rapid
secretion over the surface of the body of a thick slime which

ASEXUAL CYCLE OF PLANARIA VELATA. 195

soon hardens into a tough membrane and forms the cyst. It is
a familiar fact that stimulation is often followed in the turbellaria
by the secretion of a large amount of slime. That is exactly what
occurs in these pieces and in this species the slime hardens and
forms the cyst. Apparently then the encystment of pieces in
Planaria velata is simply the result of a sudden stimulation. Any
factor that increases the stimulation increases the frequency of
encystment.

As regards the greater frequency of encystment with advancing
age of the worms, I have not been able to reach a definite con-
clusion based on experiment, but my observations indicate that
old worms secrete more slime on stimulation than young. Ap-
parently the gland cells either increase in number or the quantity
of the substance in them which produces the slime increases as
the animals grow older.

When the slime which produces the cyst first appears it is soft
and an active whole animal is able to creep out of it without
difficulty, but the pieces are much less active and do not succeed
in escaping from it before it hardens. If the cysts are carefully
opened with needles soon after they are formed and the pieces
removed without injury or any great degree of stimulation they
usually do not encyst again but develop into whole worms
while free. But if they are injured or otherwise strongly stimu-
lated they commonly encyst a second time.

In short all the facts indicate that encystment of pieces is
merely a result of the stimulation accompanying section. It is
not an adaptation to conditions or a preparation for the future
in any sense. The animals do not encyst because they usually
live in temporary bodies of water but they are able to live under
these conditions because they encyst.

V. The Process of Fragmentation in Old Worms.

The process of fragmentation in nature is very evidently
similar in character to the process of zooid-formation and fission
in Planaria dorotocephala and P. maculata (Child, 'lie). In
consequence of increase in length of the body and the decrease
in rate of metabolism as the animal becomes older the posterior
regions of the body usually become to some extent physio-

I96 C. M. CHILD.

logically isolated from the dominant region (Child, '11a, 'lid).

That the occurrence of fragmentation is connected with a
decrease in the rate of metabolism and consequent physiological
isolatiion of posterior regions is clearly indicated by the fact that
fragmentation may often be induced, even in worms which are
not full-grown, by suddenly lowering the temperature ten to
fifteen degrees. In such cases fragmentation usually begins in
the posterior region within a few days.

The degree of isolation is not sufficient to permit development
at once into a new individual but it is sufficient to permit some
degree of independence in motor reaction, consequently, at some
time when the worm is creeping the posterior end attaches itself
and the rest of the body pulls away from it, as in P. dorotocephala.
Apparently the greater part of the body in old fragmenting
animals consists of a series of these small zooids for in most
animals fragmentation continues until only the anterior third or
fourth of the body together with the head remains. This
anterior piece may then encyst or may undergo rejuvenescence
without encystment and after some weeks give rise to a new
posterior end, or in some cases it dies.

The posterior zooids are present only dynamically and not mor-
phologically, at least not visibly, and they are not to be thought
of as absolutely fixed stable entities. When the animal is
strongly stimulated it is able to control the whole length of the
body and for the time being the posterior zooids may almost or
quite cease to exist, only to reappear after the stimulation is over.
When such zooids are established the regions at their ends must
be subjected to constantly varying correlative conditions.
Sometimes they may form a physiological posterior part of one
zooid, at other times an anterior part of another and at still
others a part of neither. Such changes in correlative conditions
must tend to weaken and eliminate the existing structure in
those regions since the development of such structure depends on
a certain degree of constancy in correlative factors. In this way
zones of structural weakness arise and these are the zones where
separation occurs.

Occasionally, either in consequence of weakness or perhaps
because the physiological isolation of the posterior regions is

ASEXUAL CYCLE OF PIANARIA VELATA.

197

insufficient the worm fails to fragment. In such cases parts of
the body may become greatly elongated and a string of connected
masses may arise. Figure 6 shows such a case.
In the posterior region four distinct masses can
be distinguished. These are connected by slen-
der bands which are merely portions of the
body greatly reduced in diameter. These four
masses are connected with the anterior portion
of the body by a long slender band resulting
from the stretching of the middle region in
consequence of the attempts of the head region
to pull away from the attached posterior parts.
These greatly elongated regions of the body
consist of little more than the body-wall and
muscles; the alimentary tract and the paren-
chyma may be almost or entirely squeezed out
of them. This animal finally became surrounded
by a cyst in the form shown in the figure, but
later the connecting strands apparently atro-
phied, the pieces became entirely separate and
each produced a whole worm.

VI. The Development of the Whole Animal Within the

Cyst.

The development of the animal from the encysted piece,
whether isolated artificially by section or by the natural process
of fragmentation, is similar in all respects to the regulatory de-
velopment of pieces which reproduce new wholes without en-
cystment. This is shown to be the case by the removal of the
cysts from pieces at various stages of the process. In all cases
the pieces are simply undergoing regulation. The process with-
in the .cyst may, however, be slower than in the unencysted piece,
probably because the supply of oxygen within the cysts is less
than in the water.

The natural method of asexual reproduction in this species
does not then differ essentially in any way from the process of
experimental reproduction. The process of fragmentation gives

198

CM. CHILD.

rise to the same conditions in the piece as experimental isolation
by section and the further history is the same in both cases.

u

f 8

Many teratological forms result from irregularities in frag-
mentation or incomplete separation. The most common are
partial duplications of anterior or posterior regions (Figs. 7 and
8) but various other forms appear. In Fig. 9, for example, a
case is shown in which an incompletely separated posterior
piece gave rise without encystment to two heads, a tail and two
outgrowths of uncertain character, and Fig. 10 shows a case in
which two worms with axes at right angles to each other are
united by the middle regions of their dorsal surfaces. Ordi-
narily the larger animal carried the other about on its back as in
the figure, the ventral surface of the smaller worm being upper-
most. Fig. 1 1 represents a case of so-called axial heteromorpho-
sis and in Fig. 12 two heads appear at the posterior end of the
larger individual and dorsal to them a tail. Evidently new

ASEXUAL CYCLE OF PLANARIA VELATA.

199

polarities arise very readily in the small pieces which result from
fragmentation, probably because the pieces are so short that the

_J-

TO

•original axial gradient (Child, '13c) is practically eliminated
and chance differences in the rate of metabolism in different parts
of the piece are sufficient to establish new polarities.

VII. Conclusion.

In Planaria velata the individual very evidently undergoes a
process of senescence as it grows and either experimental or
natural asexual reproduction brings about rejuvenescence. More-
over, the animal apparently returns to essentially the same
physiological stage with each generation, for the species is able
to persist without sexual reproduction and, as a following paper
will show, numerous asexual generations have been bred in the
laboratory without any indication of senescence of the stock.

I have shown elsewhere (Child, '\\b) that the regulation of

200

C. M. CHILD.

isolated pieces of Planaria dorotocephala brings about rejuvenes-
cence to a greater or less extent, according to the size of the piece,
the smaller piece giving rise to an animal which is physiologically
younger than that produced by a larger piece. In that species
starvation may also be a factor
in rejuvenescence. Some experi-
ments on the effect of starva-
tion on Planaria velata will be
described in another paper. At
present it need only be said that the
result is the same in both species.
In my earlier paper on senes-
cence the conclusion was reached
that senescence results from the
accumulation of structural prod-
ucts of metabolism which con-
stitute in one way or another
obstacles to the chemical reac-
tions. The processes of differentia-
tion and growth undoubtedly ope-
rate also in another way not
considered in the earlier paper,
to bring about a decrease in the
rate of metabolism per unit of
weight or volume. What we are
accustomed to call the undiffer-
entiated or embryonic cell repre-
sents the general metabolic sub-
stratum of the organism. Differ-
entiation consists in the formation and accumulation of certain
substances in the cell, some of which constitute more or less
permanent structural features. At least certain of the substances
composing these structural features are relatively stable under
the usual physiological conditions and while certain chemical
changes may occur in them, they are not broken down and elimi-
nated to so great an extent as certain other substances. This
relative stability must, in fact, be the basis of their persistence
as elements of structure. The accumulation of these structural

II

12

ASEXUAL CYCLE OF PLANARIA VELATA. 201

substances within the cell brings about a decrease in the general
metabolic activity per unit of weight or volume because it de-
creases the proportion of the material involved in the general
metabolic reactions to the inactive or less active material.
The decrease in the proportion of the general metabolic substra-
tum characteristic of the embryonic cell constitutes to some
extent a histological criterion of the physiological change in the
cell.

In short, the decrease in rate of metabolism per unit of sub-
stance, which is characteristic of development and senescence,
is undoubtedly due in part to the fact that the proportion of the
cell substance concerned in the general metabolic activity is
decreasing and the proportion of less active or relatively stable
substance is increasing. Changes in the size of the cell or in the
size relations of nucleus and cytoplasm (Minot, '08) are not
necessary factors in the result.

To what extent the decrease in the rate of metabolism during
senescence is due in a given case to actual decrease in the rate
of chemical reaction and how far to a decrease in the proportional
amount of chemically active or more active substance is often
difficult to determine, but it is probable that in some cases, or
even in some cells of the individual, the one factor and in others
the other is the more important.

As regards Planariavelata, the facts are that the rate of metab-
olism decreases during growth and development and increases
when the substances previously accumulated are removed, either
by regulatory reorganization, or by starvation. These facts
show very clearly that in one way or another the accumulation
of material in development decreases the rate of metabolism and
its removal brings about an increase in rate. Senescence and
rejuvenescence in this species consist essentially, I believe, in
these changes.

Summary.

I. After a period of growth and activity Planaria velata under-
goes fragmentation from the posterior end forward, the frag-
ments encyst and give rise by a process of regulation to whole
worms of small size.

202 C. M. CHILD.

2. During the period of growth the worms are undergoing
senescence, as the decrease in rate of metabolism indicates, but
the small worms which emerge from the cysts are physiologically,
as well as morphologically young, possess a high rate of metab-
olism and are capable of repeating the life cycle.

3. In pieces isolated by section the frequency of encystment
increases as the level of the piece becomes more posterior in the
body, with decreasing size of the piece, with rising temperature
and with increasing age of the animal. The facts indicate that
encystment is the result of stimulation. The stimulation may
result from section, from fragmentation, from a rise in tempera-
ture or from other conditions.

4. The development of the encysted piece into a new whole
animal is essentially the same process as the regulatory develop-
ment of unencysted pieces.

5. This species is able to live for an indefinite number of gen-
erations without sexual reproduction. Each new asexual genera-
tion represents a return to essentially the same physiological
and morphological stage. In other words, senescence leads to
reproduction and the process of rejuvenescence in each asexual
cycle carries the organism back to the same stage of youth.

Hull Zoological Laboratory,
University of Chicago.

REFERENCES.
Child, C. M.

'lia Die physiologische Isolation von Teilen des Organismus. Vortrage und
Aufs. ii. Entwickelungsmech., H. XL

'11b A Study of Senescence and Rejuvenescence, Based on Experiments with
Planarians. Arch. f. Entwickelungsmechanik, Bd. XXXI., H. 4.

'lie Studies on the Dynamics of Morphogenesis and Inheritance in Experi-
mental Reproduction. I. The Axial Gradient in Planaria dorotocephala
as a Limiting Factor in Regulation. Journ. Exp. ZoOl., Vol. X., No. 3.

'lid Studies, etc., II. Physiological Dominance of Anterior over Posterior
Regions in the Regulation of Planaria dorotocephala. Journ. Exp. Zool.,
Vol. XL, No. 3.

'lie Studies, etc., III. The Formation of New Zcoids in Planaria and other
Forms. Journ. Exp. Zool., Vol. XL, No. 3.

'12 Studies, etc., IV. Certain Dynamic Factors in the Regulatory Morpho-
genesis of Planaria dorotocephala in Relation to the Axial Gradient.
Journ. Exp. Zool., Vol. XII., No. 1.

'13a Studies, etc., V. The Relation between Resistance to Depressing Agents
and Rate of Reaction in Planaria dorotocephala and its Value as a Method
of Investigation. Journ. Exp. Zool., Vol. XIV., No. 2, 1913.

ASEXUAL CYCLE OF PLANARIA VELATA. 203

'13b Certain Dynamic Factors in Experimental Reproduction and their

Significance for the Problems of Reproduction and Development. Arch.

f. Entwickelungsmech., Bd. XXXV., H. 4.
'13c Studies, etc., VI. The Nature of the Axial Gradients in Planaria and

their Relation to Polarity and Symmetry. Arch. f. Etwickelungs-

mechanik, Bd. XXXVII., H. 1.
Minot, C. S.

'08 The Problem of Age, Growth and Death.
Stringer, Caroline E.
'09 Note on Nebraska Turbellaria, with Descriptions of two New Species.

Zool. Anz., Bd. XXXIV., No. 9.